This study investigates the determining parameters of thermal comfort of office in an arid hot-arid environment of Bechar, located in the northwestern region of Algeria, in which the vertical walls of the room and the roof are subjected to solar irradiations and the floor is considered to be adiabatic. The solar flux is calculated by the ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) method. The predicted results are validated against the experimental results of the meteorological station of the ENERGARID research laboratory at the University of Bechar (Algeria). The characteristics of the ambient air flow are performed by using the computational fluid dynamics (CFD) software (Fluent). The flow fields, thermal fields, and humidity are investigated. An elaborated computer program (with Delphi language) is utilized to evaluate the temperature–humidity coupling as the most essential factors of the thermal comfort. A significant impact of dynamic temperatures and humidity on thermal comfort has been observed, especially in this hot-arid environment. Besides, a considerable effect of the flow velocity has been remarked. From the obtained results and to provide the best thermal comfort in such arid regions, the range of air velocity inside the building is recommended to be between 0.2 m·s−1 and 0.3 m·s−1.